When we are called to tear out a bathroom that failed after five or six years, the tile usually still looks fine. The failure is always behind it, and it is almost always one of the same three things.
The first is a missing or incomplete waterproofing layer. Tile and grout are not waterproof; they are a wear surface. Water passes through grout by design, and something behind it has to stop that water and send it back to the drain. That is a dedicated membrane, either sheet or liquid-applied, and it has to be continuous, including up the walls and across every corner and change of plane.
The second is a shower pan with no real slope. Water needs a consistent fall to the drain, and it needs that fall in the waterproofing layer underneath, not just in the tile on top. A pan that is flat at the perimeter holds water permanently against the wall junction, which is exactly where failures start.
The third is ventilation. A fan that is undersized for the room, or ducted into the attic instead of outside, keeps the room's moisture inside the building. The result shows up as peeling paint, then as mold in the attic above, and it is genuinely common in houses where a previous remodel skipped the duct run.
There are two questions worth asking any contractor bidding your bathroom: which waterproofing system are you using and how is it detailed at the corners, and where does the exhaust fan terminate. A contractor who answers both immediately and specifically is telling you something useful about the rest of their work.
None of these items is expensive relative to the project. All of them are expensive to fix later, because fixing them means removing the tile you paid for.